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GO-2026-6605 BIT-golang-2026-56866 CVE-2026-56866

HTTP/1 client connection desynchronization after CONNECT rejection in net/http

High Go
Severity
7.5 / 10
High · CVSS 3.1
Exploit chance (EPSS)
0.47%
Chance of exploitation in the next 30 days. Higher than 40.98% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-444 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
Fix
1.27.2
Upgrade stdlib to this version or later

Description

When http.Transport sends an HTTP/1 CONNECT request with a non-empty Request.Body, it writes the body directly to the connection without framing after the request headers. If the server rejects the CONNECT request with a non-2xx keep-alive response, Transport returns the connection to the idle pool. Because CONNECT requests do not have a request body, the server may interpret the trailing body bytes as a subsequent pipelined HTTP/1.1 request on the connection, leaving the pooled connection desynchronized and causing the next caller that reuses it to read the response to the injected request. In reverse proxies (including httputil.ReverseProxy) that forward CONNECT requests through a shared Transport, this can lead to cross-user response poisoning.

Affected versions

Go

PackageVulnerableFixed in
stdlib
< 1.26.9
>= 1.27.0-0, < 1.27.2
1.26.9
1.27.2

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

stdlib Go → 1.27.2

go
go get stdlib@v1.27.2 && go mod tidy

Severity breakdown

CVSS 3.1 7.5

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

Weakness

CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.

How to prevent it
  • Implementation: Use a web server that employs a strict HTTP parsing procedure, such as Apache [REF-433].
  • Implementation: Use only SSL communication.
  • Implementation: Terminate the client session after each request.
  • System Configuration: Turn all pages to non-cacheable.

References

Fixes and patches

Issue reports

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
GHSA-4374-p667-p6c8
HTTP/2 rapid reset can cause excessive work in net/http
High Known exploited Also affects stdlib
CVE-2020-0601
Certificate validation bypass on Windows in crypto/x509
High Known exploited Also affects stdlib
RUSTSEC-2026-0382
Distrust compromised intermediate certificates in crypto/x509
Medium Also affects stdlib
GO-2026-6613
HTTP/1 server connection desynchronization after 2xx CONNECT response in net/http
High Also affects stdlib
GO-2026-6609
Lack of limit on size of parsed Range headers in net/http
High Also affects stdlib
GO-2026-6608
Memory limit bypass when parsing MIME headers in net/textproto, mime/multipart
High Also affects stdlib
GO-2026-6607
Reject malformed ECH outer extension references in crypto/tls
High Also affects stdlib
GO-2026-6604
Root.Mkdir(All) can follow junctions out of the root on Windows in os
Critical Also affects stdlib

Credits: Xclow3n (Rajat Raghav)

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